Circuit protection and bidirectional electrical isolation are strongly desirable attributes of electrical circuits likely to be exposed to outside stresses as well as to electrical circuits with the potential to introduce hazards. Examples of outside stresses include electrostatic discharge (ESD) events from such sources as lightning, other natural events, and human contact. Examples of hazards potentially able to be introduced by electronic circuits include interference with medical test results and causation of sparks that can ignite flammable mixtures such as medical gases mixed with air or oxygen.
One area of particular interest is ESD events that occur in nominally sheltered areas such as buildings, and particularly those occurring in locations such as hospitals and other care facilities. ESD is caused by separation of similar materials, such as peeling a strongly insulating material like polyethylene film from a roll; by repeated rubbing between differing materials, such as the classic rabbit fur and amber rod demonstration; and by many everyday events such as walking across a linoleum floor in leather-soled shoes. ESD is gauged by voltage and available energy storage, and by such factors as the humidity-dependent conductivity of air, which factors contribute to a determination of the distance a spark can leap.
ESD events are diminished by bleeding off static charge through ameliorative strategies, such as replacing static-promoting furniture, clothing, bedding, and carpets with dissipative types, applying carefully formulated waxes to linoleum floors, adding extra water to dry air, and the like. However, the complete elimination of ESD is seldom practical. A balanced approach to ESD control combines amelioration strategies with electronic circuit designs that increase immunity of circuits to damage due to those ESD events that are not successfully suppressed.
Several ESD models exist to permit and verify successful apparatus design for the prevention of damage. Typical ESD models include combinations of voltage levels, energy storage levels (capacitance), and pulse rise time to emulate one or more modes analogous to such events as a human walking across a static-promoting floor in dry weather and touching a test subject device with a finger. Since established ESD models generally use voltage levels well below 100 kilovolts (KV), that level is a useful threshold for developing an apparatus that is substantially ESD resistant.
In general, circuit designs that have heightened intrinsic ESD immunity are desirable in comparison to lower ESD immunity circuits. Tradeoffs between such devices in order to enhance survivability in normal use involve such issues as higher power consumption, slower response time, diminished accuracy, and higher cost. Thus, any circuit improvement that can potentially improve ESD immunity with minimal penalties may be viewed with great interest.
In particular, in a hospital setting, a human interface device such as a call cord apparatus may be viewed as a candidate for improved ESD protection. A call cord is a device used at a hospital bed and elsewhere, typically electrically connected to a wall-mounted electrical enclosure. The call cord, itself, is, in some embodiments, a wire pair ending at the wall end in a plug and at the opposite end in a handle with a pushbutton switch. A call cord is typically used by a patient to request the attention of an attendant. Because the call cord contains conductive components and is covered in insulating material, adding a requirement for ESD dissipative qualities is an undesirable imposition on the design of the call cord itself.
A further call cord restriction that requires consideration is the possibility that the call cord itself could be a source of physical hazard. A spark of any size occurring exterior to or within a call cord, such as a spark associated with a switch closure, is potentially capable of causing electrical interference with devices in its immediate vicinity. In addition, an improperly configured call cord could be a source of undesirable arcing.
Accordingly, it is desirable to provide a method and apparatus that allow a call cord-type switch for use in a hospital grade wall-mount signal annunciator to operate in an essentially normal mode, while establishing an ESD immunity threshold on the order of 100 Kilovolts and providing substantial immunity from spark generation.